SuGoiSuす語彙すGrammar › N1 · 01 Conditions and hypotheticals
〜が最後: once ... that's the end
N1 · 01 Conditions and hypotheticals
Means that once an action happens, a fixed bad outcome follows with no escape.
Verb た-form + が最後, followed by a result that is hard to reverse. Stronger and a little more literary than 〜たら最後. It is used for facts, not for requests or orders, in the second half.
Examples
その店に入ったが最後、財布が空になるまで出られない。
Once you go into that shop, you can't get out until your wallet is empty.
彼は一度ゲームを始めたが最後、朝まで止められない。
Once he starts a game, he can't stop until morning.